Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/themes/seven/js/mobile.install.js

@@ -0,0 +1,33 @@
+(function () {
+
+  "use strict";
+
+  function findActiveStep(steps) {
+    for (var i = 0; i < steps.length; i++) {
+      if (steps[i].className === 'is-active') {
+        return i + 1;
+      }
+    }
+    // The final "Finished" step is never "active".
+    if (steps[steps.length - 1].className === 'done') {
+      return steps.length;
+    }
+    return 0;
+  }
+
+  function installStepsSetup() {
+    var steps = document.querySelectorAll('.task-list li');
+    if (steps.length) {
+      var header = document.querySelector('header[role="banner"]');
+      var stepIndicator = document.createElement('div');
+      stepIndicator.className = 'step-indicator';
+      stepIndicator.innerHTML = findActiveStep(steps) + '/' + steps.length;
+      header.appendChild(stepIndicator);
+    }
+  }
+
+  if (document.addEventListener) {
+    document.addEventListener('DOMContentLoaded', installStepsSetup);
+  }
+
+})();